1. Field of the Invention
The present invention refers to a control circuit for current flowing in step motor phase windings.
2. Description of the Prior Art
Current control systems of the so-called current chopping type are known; they permit high performances in step motors by keeping the energization current in the phases close to predetermined values for the whole energization time interval of such phases. Substantially such systems connect intermittently the phase windings to a voltage source, interrupting the connection when the current reaches a predetermined value and allowing the current established in the winding to flow in a recycle path, thus decreasing with a time constant determined by the recycle path impedance which connects the windings again to the voltage source after a predetermined interval, or when the circulating current has decreased to a second predetermined value. This process continues for the whole duration of the phase's energization. Such systems are described for example in U.S. Pat. Nos. 4,107,593 and 3,812,413.
Within improved systems of the same type the predetermined value for the energizing current may be varied according to the operation requirements. When a high energizing power is needed, as for example during the motor starting or stopping phases, the imposed current level is high. On the other hand during the motion phase at constant speed or when the energizing current function is only to hold the rotor in a stable position, the current level is low. An example of such a system is described in the Italian patent application No. 24734 A/80 of the same applicant as the instant application. The current chopping control system, used in step motors, have the disadvantage of causing motor vibrations with a frequency determined by the "chopping" frequency. This is due to the intermittance of the electrodynamical actions in the motor electromagnetic structures. If the "chopping" frequency utilized is within the audio-frequency range electrical noise is generated. In practice, nonetheless, the motor is required to operate within such frequency ranges. In fact for very low "chopping" frequency, in the range of a few tens of Hz, the control action on the energizing current is ineffective; while for frequency higher than 15 KHz the switching losses in the switching devices (generally power transistors) become so important as to greatly reduce the performances of the whole control system and to require the use of expensive switching transistors able to dissipate the switching power losses.
The present invention overcomes such disadvantages by using a chopping criterion in which the chopping frequency is selected as an inverse function of the predetermined value of the energization current. In fact, it is known that the switching power losses in switching transistors are substantially: EQU P=K.multidot.I.multidot.f.multidot.v.
Where
K=proportion coefficient PA1 I=switched current PA1 f=switching frequency PA1 v=switched voltage. PA1 a. the rational use of switching transistors within safety conditions and with constant dissipated power; PA1 b. the possibility of using low cost transistors and simple and inexpensive control circuits; PA1 c. the possibility of operating at least within working conditions of the step motors, at ultrasonic switching frequencies, so that unwanted noise is eliminated.
Once the maximum power to be dissipated is established, it is possible to vary f inversely to current while keeping I and P within the allowed limits. Several advantages are obtained by the use of such criterion. They are as follows:
These advantages are particularly useful in the field of mosaic serial printers where step motors are widely used. In such printers a step motor is used to move a printing carriage. The use of step motors allows the carriage to be stopped at predetermined holding positions and held firmly in such positions, and to intermittently move from a holding position to another one by accelerating, driving at constant speed and decelerating. During the displacement from a holding position to another one, the printing operations take place. The printing operations involve noise generation. Such noise, with predominant frequency in the order of thousand Hz, exceeds and masks any noise generated by electrodynamical actions within the step motor. However, when the carriage is in holding state, no printing operation occurs and the electrodynamic actions within the step motor cause a noise with commutation frequency of about some thousands Hz (5-6 KHz). This is detected by the operator as a very annoying whistle.